Bioavailability and Resupply Dynamics of Thallium in Agricultural Soils Affected by Lead-Zinc Mining: In-Situ Insights from DGT-DIFS Modeling

Abstract

Thallium (Tl), with higher toxicity and bioaccumulation potential than mercury (Hg), cadmium (Cd), and lead (Pb), poses serious ecological and food safety threats in mining-impacted agricultural systems. Despite this, its in-situ bioavailability and desorption kinetics in soil remain poorly understood, particularly in lead-zinc mineral-rich regions of southwestern China. In this study, we combined the Diffusive Gradients in Thin-Films (DGT) technique, the DGT-Induced Fluxes in Soils (DIFS) model, and path analysis to comprehensively investigate Tl mobility, bioavailability, and its environmental controls. A total of 29 soil samples from lead-zinc mining, smelting, and processing zones were analyzed. Over 59% of the soil samples exceeded the 1 mg/kg Tl threshold adopted in Germany, Canada, and Switzerland, with Tl enriched in ∼200 nm soil particles. DGT-measured labile Tl concentrations ranged from 0.03 to 1.71 μg/L, reaching peak levels in smelting areas, where weak acid-extractable Tl accounted for 2.9%-22.2% of the total. DIFS modeling revealed rapid resupply of Tl from the solid phase in contaminated soils (T_c = 15.9 s), indicating high environmental mobility. Path analysis identified total soil Tl content and cation exchange capacity as the dominant direct factors controlling Tl bioavailability, with standardized path coefficients of 0.49 and -0.33, respectively. Notably, manganese oxides indirectly enhanced Tl bioavailability (indirect coefficient = 0.37) by facilitating Tl accumulation and release, highlighting their dual role in both pollutant transport and potential remediation. This study provides novel mechanistic insights into Tl behavior in contaminated soils and demonstrates the strength of DGT-DIFS as a robust in-situ tool for evaluating heavy metal risks and informing soil pollution management strategies.